Among 2,2'-azoimidazole derivatives currently known is 1,1'-bisbenzylideneamino-4.4'-diphenyl-2,2'-azoimidazole described in FRG Journal (Chem.Ber., vol.101, September 1968, Verlag Chemie, Valgheim, Beyer A., Hetzheim, H. Honeck, D. Linq, T. Pyl "Synthesis of Novel Imidazole Derivatives", p.3151-3162).
However, a utility of the above-mentioned compound has not been described anywhere.
A great number of dyestuffs are known in the art for mesomorphic materials featuring a positive dichroism (cf. U.S. Pat. No. 3,703,329 published 21.11.1972, Cl. 350-150). At a planar orientation of the mesomorphic material, e.g. with a positive anisotropy, incorporating a dyestuff with positive dichroism (S above O) the working field in a electrooptical device looks coloured. If a voltage applied to the electrooptical device is above some critical value, the orientation of the mesomorphic material becomes homeotropic and the working field looks faintly coloured. Dyestuffs with a negative dichroism under the above-specified conditions give a reverse picture and a combination of dyestuffs with positive and negative dichroism makes it possible to obtain colour shifting in the electrooptical device, provided that these dyestuffs have spectrally spaced absorption bands within the visible region of spectrum.
While as it has been mentioned hereinbefore, dyestuffs with a positive dichroism are known in a sufficiently broad range, dyestuffs with a negative dichroism which are of a practical interest are limited to tetrazine derivatives (cf.French Pat. Nos. 2,416,253; 2,422,707, 2,428,666 Cl. C 09 K 3/34, published 1978, 1978 and 1979 respectively).
Among the dyestuffs for mesomorphic materials having two absorption bands in the visible region of the spectrum with dichroism of opposite signs, only bismerocyanine is known which has an absorption band with the maximum at 574 nm (S above O) and an absorption band with the maximum at 482 nm (S below O) (cf. Doklady Akademii Nauk SSSR, vol. 220. February 1975, "Nauka" Publishing House, Moscow: L. M. Blinov, G. G. Dyadyusha, F. A. Mikhailenko, I. L. Mushkalo, V. G. Rumyantsev "Polarization of absorption bands of Solutions of Biscyanine Dyes in Liquid Crystals", pp. 860-862). However, this dyestuff has, on the one hand, absorption bands located in the visible area of spectrum such that their position does not ensure the contrast required for a human eye. Furthermore, this dyestuff makes it possible to obtain, in an electrooptical device, light signs against a dark background which is also unsatisfactory for perception by a human eye.
Dyestuffs for mesomorphic materials having two spectrally spaced absorption bands within the visible area of spectrum, so that the long-wave band has a negative dichroism and the short-wave band has a positive dichroism, are not known at all.
Due to the lack of dichroic dyestuffs of the latter type there is also lack of appropriate mesomorphic materials and corresponding electrooptical devices.